diff --git a/src/ert/config/design_matrix.py b/src/ert/config/design_matrix.py
index 52bc354382e..7a17f8f3919 100644
--- a/src/ert/config/design_matrix.py
+++ b/src/ert/config/design_matrix.py
@@ -141,26 +141,23 @@ def read_design_matrix(
         # Read the parameter names (first row) as strings to prevent pandas from modifying them.
         # This ensures that duplicate or empty column names are preserved exactly as they appear in the Excel sheet.
         # By doing this, we can properly validate variable names, including detecting duplicates or missing names.
-        try:
-            param_names = (
-                pd.read_excel(
-                    self.xls_filename,
-                    sheet_name=self.design_sheet,
-                    nrows=1,
-                    header=None,
-                    dtype="string",
-                )
-                .iloc[0]
-                .apply(lambda x: x.strip() if isinstance(x, str) else x)
-            )
-            design_matrix_df = DesignMatrix._read_excel(
+        param_names = (
+            pd.read_excel(
                 self.xls_filename,
-                self.design_sheet,
+                sheet_name=self.design_sheet,
+                nrows=1,
                 header=None,
-                skiprows=1,
+                dtype="string",
             )
-        except (ValueError, AttributeError) as exc:
-            raise ValueError(f"Error reading design matrix: {exc}") from exc
+            .iloc[0]
+            .apply(lambda x: x.strip() if isinstance(x, str) else x)
+        )
+        design_matrix_df = DesignMatrix._read_excel(
+            self.xls_filename,
+            self.design_sheet,
+            header=None,
+            skiprows=1,
+        )
         design_matrix_df.columns = param_names.to_list()
 
         if "REAL" in design_matrix_df.columns:
diff --git a/src/ert/run_models/ensemble_experiment.py b/src/ert/run_models/ensemble_experiment.py
index 1b51125bc24..2372da969eb 100644
--- a/src/ert/run_models/ensemble_experiment.py
+++ b/src/ert/run_models/ensemble_experiment.py
@@ -77,8 +77,6 @@ def run_experiment(
             except ConfigValidationError as exc:
                 raise ErtRunError(str(exc)) from exc
 
-            assert design_matrix.active_realizations is not None
-            self.active_realizations = design_matrix.active_realizations
         if not restart:
             self.experiment = self._storage.create_experiment(
                 name=self.experiment_name,
diff --git a/src/ert/run_models/model_factory.py b/src/ert/run_models/model_factory.py
index 68cb4a75d56..f0688309201 100644
--- a/src/ert/run_models/model_factory.py
+++ b/src/ert/run_models/model_factory.py
@@ -117,13 +117,20 @@ def _setup_ensemble_experiment(
     args: Namespace,
     status_queue: SimpleQueue[StatusEvents],
 ) -> EnsembleExperiment:
-    active_realizations = _realizations(args, config.model_config.num_realizations)
+    active_realizations = _realizations(
+        args, config.model_config.num_realizations
+    ).tolist()
+    if (
+        config.analysis_config.design_matrix is not None
+        and config.analysis_config.design_matrix.active_realizations is not None
+    ):
+        active_realizations = config.analysis_config.design_matrix.active_realizations
     experiment_name = args.experiment_name
     assert experiment_name is not None
 
     return EnsembleExperiment(
         random_seed=config.random_seed,
-        active_realizations=active_realizations.tolist(),
+        active_realizations=active_realizations,
         ensemble_name=args.current_ensemble,
         minimum_required_realizations=config.analysis_config.minimum_required_realizations,
         experiment_name=experiment_name,
@@ -271,9 +278,9 @@ def _setup_iterative_ensemble_smoother(
         random_seed=config.random_seed,
         active_realizations=active_realizations.tolist(),
         target_ensemble=_iterative_ensemble_format(args),
-        number_of_iterations=int(args.num_iterations)
-        if args.num_iterations is not None
-        else 4,
+        number_of_iterations=(
+            int(args.num_iterations) if args.num_iterations is not None else 4
+        ),
         minimum_required_realizations=config.analysis_config.minimum_required_realizations,
         num_retries_per_iter=4,
         experiment_name=experiment_name,